An organic light emitting diode (OLED), which is also referred to as organic EL (electroluminescence), is a light emitting device utilizing the principle that a fluorescent material emits light by recombination energy of a hole injected from an anode and an electron injected from a cathode when an electric field is formed by the organic (low molecular weight or polymer) semiconductor of a conjugated structure as a light-emitting material inserted between both electrodes. Such an OLED is the latest high-tech display device that has been widely used for outer and inner windows of a cellular phone, displays of an MP3 and a digital camera, and so forth in recent years, and is expected to be also applicable to a wall-mounted type TV and a flexible TV succeeding PDP and LCD TVs due to low power consumption, a high response speed, and a wide viewing angle.
Various kinds of light emitting device materials comprising organic materials have been developed since a low-voltage-driven OLED using a laminated structure was reported by C. W. Tang et al. of Eastman Kodak Co. in 1987 (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, Vol. 51, p. 913, 1987), and an OLED with a double structure including two organic layers disposed between an anode and a cathode was proposed in U.S. Pat. No. 4,356,429.
An OLED includes a cathode, electron injecting/transporting layers (EIL/ETL), a light emitting layer (EML), hole injecting/transporting layers (HIL/HTL), an anode, a substrate, and the like, and particularly the EML comprises an organic material. One of typical compounds used as the EML material is Alq3, and much research has been done on quinoline complex derivatives, in the course of which some blue light emitting layer materials, such as DPVBi, etc., were also discovered.
A dopant is a material that has a great effect on improving luminous efficiency, but there has been a difficulty in developing the dopant because of insufficient luminous efficiency, discordance with a host material, and so forth. In particular, a blue dopant is difficult to increase purity because a styryl structure is not thermally stable, and thus there is a problem in that color purity or efficiency is lowered or lifetime is shortened. Also, in order to provide deep blue color purity, a band gap between the HOMO and the LUMO of a dopant must be large, but it is not easy to develop such a dopant material.